1. Technical Field
The present invention generally relates to a moisture sensor for moisture-activated windshield wiper control systems. More particularly, the present invention is directed to an electronic circuit and technique for testing the operation of the moisture sensor.
2. Discussion
Moisture activated windshield wiper systems are increasingly being incorporated into automobiles as standard equipment. Windshield wiper systems of this type typically include an optical sensor (infrared or visible spectrum) which is aimed at or mounted to the windshield, and electronic circuitry including a processing circuit for processing signals from the optical sensor to detect the presence of water droplets or moisture on the windshield. In operation, these systems direct light energy toward the outside surface of the windshield through an optical path. The optical path includes a photoemitter, a photodetector, and a quantity of radiation internally reflected at the windshield/air interface on the windshield surface. The rain sensing processing circuit generally includes electrical gain, frequency filtering, and other means to discriminate the desired rain stimulus. The output of the processing circuit controls the wiping frequency and speed of the windshield wiper motor in response to the amount of water detected on the windshield surface. A windshield wiper system of this type is shown in U.S. Pat. No. 5,059,877 to Teder, the disclosure of which is incorporated herein by reference.
The presently known rain sensors typically include one or more radiant energy sources such as infrared LED's, and optical means such as a collimating lens, a detector or focusing lens, and photodetector receivers to form a target area on the windshield surface which is sensitive to water droplets or moisture. A system of this type is described in U.S. Pat. No. 4,620,141 to McCumber et al. The McCumber patent described a moisture sensor system where a water droplet within the target area results in a change in the effective angle at which the light strikes the air/water surface. This operation is based upon the principle that air has a different refractive index than water and allows more of the light to escape from the windshield surface.
The above referenced Teder patent teaches that the strength of the signal returning to the photodetectors may be detected, and this detected signal may be used for appropriately changing the intensity of the photoemitters, allowing the system to function as a constant signal-to-noise device. The method of the Teder patent further teaches a rain sensing control circuit that provides for automatic LED current adjustment to compensate for changes in the transmittance of different windshields. The Teder system provides this automatic LED adjustment by regulating the average current in the photodetector devices. The Teder system also provides an automatic rain sensing system that is self-calibrating, allowing use on windshields having varying optical transmissivity characteristics.
However, as part of the Teder system, the current in the individual photodetectors will not be identical due to variation in optical and electrical gain parameters. Thus, the area and sensitivity to rain of the target areas will vary from channel to channel. As will be appreciated, a problem occurs if one or more of the optical target areas varies greatly in sensitivity or gain relative to the other optical target areas. When this problem condition occurs, the circuit taught by Teder will adjust the variable intensity pulse generator that drives the photoemitters, to the point where the average current though the photodetectors is equal to a level set by a predefined reference voltage. Thus, the average current in the photodetectors is regulated to a reference level, but the current in each individual photodetector will vary. The individual optical target areas corresponding to these photodetectors will therefore vary in their sensitivity to rain. For example, if the rain sensing system consists of four optical target areas and two of the target areas are completely nonfunctional, the Teder circuit will increase the current in the remaining two optical target areas, greatly increasing their sensitivity, in an effort to compensate for the two nonfunctional target areas. However, this technique can only partially compensate for the loss of the two targets, since the available target area for detecting water droplets will be greatly reduced in a system with two nonfunctional target areas.
In view of the above problems associated with the prior art rain sensors, it is desirable to provide a moisture sensing system which includes a self test feature for accurately determining whether the individual optical target areas are functioning properly. It is also desirable that this self test feature is integrated within the existing moisture sensing circuitry. It is further desirable that the moisture sensor have the capability to notify either the vehicle driver or service technician that the moisture sensing system is not functioning properly.